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December, 1981 



Mosqwto News 



751 



AN ACOUSTIC EVALUATION OF ELECTRONIC 
MOSQUITO REPELLERS 

P. BELTON 

Pestology Centre, Department of Biological Sciences, Simon Fraser University, Burnaby, B.C. 

V5A 1S6, Canada 



ABSTRACT. An analysis of the acoustic 
output of 4, commercial devices claimed to 
repel mosquitoes showed fundamental fre- 
quencies between 1.95 and 5 kHz and many 
harmonics extending into the ultrasonic range. 

INTRODUCTION 

The development and marketing of 
several small sound generators intended 
to repel female mosquitoes followed the 
publication in an electronic hobbyist's 
journal of an article entitled "Build the 
Bug-Shoo" (Greenlee 1970). This 
pocket-sized device was claimed to be ef- 
fective because "The female mosquito is 
thought to be repelled by the same sound 
that attracts the male," The sound that 
attracts the male, in species that mate in 
swarms, is generated by the wing move- 
ment of the female (Roth 1948, Belton 
and Costello 1979) and although this 
sound is complex, only the fundamental 
frequency of the wingbeat — a sine 
wave — is essential to attract males 
(Wishart and Riordan 1959). 

Another statement in the article is that 
"the male mosquito is attracted by a 
humming noise at a frequency of 2000 
Hz" (cycles per second). This is 
incorrect — the males of most pest species 
in North America are attracted by fre- 
quencies of 200-350 Hz, the wingbeat 
frequencies of the corresponding females 
(Belton 1967, Belton and Costello 1979). 
Set at the frequency he suggests, 
Greenlee's device produces a sound 6-10 
times too high to duplicate the buzz of a 
female. 

The Bug-Shoo, a unijunction-transistor 
oscillator driving a miniature earphone, 
was built in 1972. Its output waveform 
consisted of a train of pulses that could be 
varied over a wide range of frequencies. 
Its sound is, therefore, complex and 



None of them had significant repellent effects 
on laboratory bred Aedes aegypti or Cukx pipiens 
or on field populat ions, mostly of At. hexodon- 
tus. 

contains harmonics whose frequencies 
depend on the pulse rate. Because it 
could be tuned over a wide range of fre- 
quencies a comprehensive analysis of this 
device was not practical. 

The 4 commercial sound generators 
tested all had fixed frequency outputs. 
Advertising for one of them claimed that 
its output contained ultrasonic frequen- 
cies that simulated an echolocating bat 
and that this would repel biting insects 
(Costello and Brust 1976). Another 
generator Was claimed by its manufactur- 
ers to simulate the buzz of a mate mos- 
quito which they suggested would repel 
mated females. Three of the generators 
appear to be based on the unijunction 
oscillator of Greenlee and therefore 
should have pulsed outputs but the only 
descriptions in the literature of the 
acoustic output of commercial mosquito 
"repellers" stated that they were sine 
waves (Kutz 1974, Singleton 1977). The 
following analysis of the output of 4 of 
them was therefore carried out. 

Because the American Mosquito Con- 
trol Association is still receiving enquiries 
about such devices (T. D. Mulhern, 
Executive Director, personal communi- 
cation) tests of their effectiveness were 
made and their potential is discussed in 
relation to claims for their mode of ac- 
tion. 

MATERIALS AND METHODS 

Three devices were tested both in the 
laboratory and in the field. They are 
identified as follows: A, "electronic 



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Mosquito News 



Vol. 41, No. 4 



gadget electronic mosquito repeller," 
supplied by Murray Distributors Ltd., 
North Vancouver B.C.; B, "Electronic 
Mosquito Repeller" obtained from Peak 
Distributions Ltd., Surrey B. C. and C, 
"Moziquit" supplied by Electronic Pest 
Controls Ltd., Montreal P.Q. All are 
roughly rectangular 6x5x2 cm deep 
plastic boxes with a transistor-radio ear- 
phone at the bottom right of the front 
face. The device claimed to sound like a 
male, D, is currently being marketed in 
Canada (June 1981) by Canaco Market- 
ing, Toronto, Ont. with the name "Anti- 
bite." It was apparently also available in 
Europe under the name "Antipic." It is 
cylindrical, about 7x2.5 cm in diameter 
with the earphone facing upward at the 
top of the cylinder. The sound level from 
each was measured using a Bruel and 
Kjaer type 2204 impulse precision sound 
level meter with a 4145 condenser mi- 
crophone, at a distance of 1 cm. The 
amplified output of the meter was fed 
into a Tektronix type 3L5 spectrum ana- 
lyzer to determine the frequency content 
of the sound in the range between 100 Hz 
and 50 kHz. An oscillogram of the sound 
and its spectrum were photographed si- 
multaneously from a Tektronix type 556 
dual beam oscilloscope. 

The effectiveness of the 4 devices was 
investigated using screen cages, 
30x30x60 cm high, with a sleeve of 15 
cm dia in the Plexiglas front. Eight 1-min 
tests were carried out each with 100 pre- 
viously untested female Aedes aegypti 
(Linnaeus) and five 5-min tests each with 
50 untested female Culex pipiens Linnaeus. 

The tests on Ae. aegypti were run with 
one of the devices on the floor of a cage. 
Its sound was transmitted upwards when 
it was switched on and it was off in alter- 
nate tests. All the mosquitoes landing on 
and probing a human hand were counted 
during each test. The tests on Cx. pipiens 
used a restrained white rat and counts 
were made during the 5-min tests of each 
device. The counts of Cx. pipiens were 
compared with a single control test with 
no device in the cage. 

Field tests of the devices available in 



1977 (A, B and C) were carried out at 6 
different sites in mountain woodland 
using 5-min counts of landings on 2 
seated subjects, each with an exposed 
forearm. Alternate counts were made 
with the devices switched on and off. Two 
counts (first on then off) were made with 
device A and 2 with device B. Because 
these tests had little effect on biting rates 
a final two counts were made with devices 
A, B and C switched on together and then 
off. The results were analyzed with the x 2 
test and the significance determined at 
the 1% level (P<0.01). Because of the low 
numbers landing, Yates' correction was 
applied to the results obtained from Cx. 
pipiens. 

RESULTS 

The output of the 4 commercial devices 
is shown in Fig. 1, A to D. The left-hand 
traces are accurate representations of 2 
msec of the sound from each device in the 
audible frequency range. The right-hand 
traces are spectral analyses showing a 
linear representation of sound pressure 
(vertical scale) at frequencies between 
and 50 kHz (horizontal scale). The sen- 
sitivity of the microphone is gready de- 
creased above 20 kHz so that the vertical 
calibration does not hold good for the 
right half of the spectra. The spectra 
show a series of peaks. The peak at zero 
frequency is an artifact generated by the 
analyzer. To its right is the low-amplitude 
fundamental frequency of the device (ar- 
rowed) and succeeding peaks are those of 
the second (asterisked) and higher har- 
monics, each of which is linearly related 
to the sound pressure at that frequency. 
Each sound had many harmonics at mul- 
tiples of its fundamental frequency, re- 
lated to the pulsed output of its oscillator. 
The less obvious pulsing of device D gives 
rise to fewer harmonics and its output in 
the audible range is mostly the second 
harmonic at 10 kHz. As the distribution 
of harmonic peaks indicates, both pitch 
and quality (timbre) of all 4 sounds are 
audibly different from each other. The 
right-hand side of the spectra in Fig. 1 



December, 1981 



Mosquito News 



753 




5 10 15 20 



30 



40 



Frequency, kHz 

Fig. 1. Left traces, oscillograms of 2 msec of the sound from devices A, B, C arid D, re- 
spectively. Right traces, spectral analyses of sounds at left; fundamental frequency arrowed, 
second harmonic with an asterisk and higher harmonics to the right. The vertical calibration 
represents 50 mV RMS output from the spectrum analyzer showing reduced amplification for 
traces B and D. 



show slight deflections which indicate that 
ultrasonic frequencies are present in the 
output of all 4 devices. 

The sounds are not particularly in- 
tense, their pressure, at a distance of 1 
cm, and their fundamental and highest 
amplitude harmonic frequencies are 
shown in Table 1. 

Table 1. Characteristics of the sound of the 
4 devices tested. 



Device 


Sound Fundamental 
pressure frequency 


Peak 
harmonic 


A 


74 dBA 2.0 kHz 


4 kHz 


B 


68 2.5 


15 


C 


84 3.0 


9 


D 


72 5.0 


10 



Sounds of this intensity are not audible 
at distances of more than about 1 m, over 
the background noise of a "suburban liv- 
ing room" (Broch 1969). Nevertheless 
they are about 50- 100 times louder than - 
the sound of a female mosquito which is 
about 40 dBA at 1 cm (Wishart and Rior- 
dan 1959) and they are several hundred 
times less intense than that of an 
echolocating bat which is about 100 dB at 
30 cm (Belton and Kempster 1962). 

Between 70 and 87 of the caged Ae. 
aegypti landed and were probing the ex- 
posed hand at the end of each 1-min test 
and the numbers were not significantly 
different whether any of the devices was 
on or off. Between 3 and 14 of the 50 Cx. 
pipiens landed and probed the rat. In the 



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Vol. 41, No. 4 



test of device B, fewer mosquitoes landed 
when the sound was on, however, the x 2 
value (with Yates' correction) of 2.13 indi- 
cates that this could have occurred by 
chance in 1 trial out of 10. In the field 
tests there were no significant differences 
in numbers landing in 5-min periods 
whether one or more of the devices was 
switched on or off. The landing rates on 
one arm were between 2 and 7/min. Mos- 
quitoes were mostly Ae, hexodontus Dyar, 
with a few Ae. cataphylla Dyar and Culiseta 
inornata (Williston). 

DISCUSSION 

The acoustic waveforms of devices A, B 
and C are pulsed like that of the Bug- 
Shoo and are consistent with a unijunc- 
tion oscillator. Kutz (1974) and Singleton 
(1977), both investigated device B and de- 
scribed its output as a sine wave, however, 
they may not have used sufficiently pre- 
cise equipment to describe the waveform 
accurately. The acoustic characteristics of 
transistor-radio earphones are variable 
even from the same manufacturer and it 
is probable that the amplitude and 
quality of the sounds of some of the de- 
vices vary from unit to unit but this was 
not investigated. None of these 3 devices 
correspond in frequency with the flight 
sounds of female North American mos- 
quitoes. Certainly they could not be ex- 
pected to work on the principle of repel- 
ling females with the same sound that at- 
tracts males. Device D, which was de- 
signed to mimic the sound of a male mos- 
quito, on the hypothesis that it would 
repel mated females, produces a fre- 
quency 10 or more times higher than that 
of any North American male mosquito 
tested. A preliminary investigation of the 
antennae of females indicates that they 
are less sensitive to sound than those of 
males and are tuned to a frequency 
around 100 Hz, a fiftieth of the funda- 
mental frequency of device D. Sound at 
the wing beat frequency of the male, loud 
enough to be heard by the females, would 
probably be as annoying to humans as the 
bites themselves. 



The results of the laboratory and field 
tests confirm those of previous inves- 
tigators (Garcia etal. 1976, Gorham 1974, 
Helson and Wright 1977, Kutz 1974, 
Rasnitsyn et al. 1974, Schreck et al. 1977, 
Snow 1977). The devices had no consis- 
tent repellent effect. The 3 species en- 
countered in the field were different 
from those investigated in previously 
published field tests but were similar in 
their lack of response. The unpleasant 
nature of the 4 sounds, as perceived by 
the human ear, may be due to the pres- 
ence of some ultrasound in the output of 
all the devices tested but the negative re- 
sults of all laboratory and field tests indi- 
cate that ultrasound at these levels is, tike 
the audible component, not significantly 
repellent to mosquitoes. Greenlee's Bug- 
Shoo can be tuned to an inaudibly high 
frequency and he states that "all mos- 
quitoes seem to be repelled by a high fre- 
quency, above 10,000 Hz." Mosquitoes 
may have mechanoreceptors sensitive to 
high frequencies but no physiological re- 
search has been done to locate them. 
There is, at present, no scientific proof 
that the behavior of female mosquitoes is 
affected by any sound frequency but this 
should not discourage further research 
into the effect of ultrasonic frequencies 
and also of frequencies lower than that of 
the wingbeat, to which the antennae of 
female mosquitoes are sensitive. 

I can find no scientific basis for any of 
the claims made for the mode of action of 
any of the repellers that have been 
marketed up to the present. On the con- 
trary, most observers exposed to high 
biting populations of mosquitoes know 
that, although they may jostle for posi- 
tion, flying or walking mosquitoes do not 
repel each other. My experience of 
working near swarms of male mosquitoes 
is that their sound does not deter females 
from biting and in fact there are several 
species in which both males and females 
are attracted to hosts with no mutual re- 
pulsion. 

CONCLUSIONS 
All 4 putative insect repellers were inef- 



December, 1981 



Mosquito News 



755 



fective in laboratory and field tests against 
the aforementioned mosquito species al- 
though their frequencies extended well 
into the ultrasonic range. Because of their 
high fundamental frequency, hone of 
them could work, as Greenlee (1970) sug- 
gested they do, by reproducing a sound 
that attracts male mosquitoes. Only one 
sample of each device was tested. 

Acknowledgment. I thank Dr. H. R. 
MacCarthy for his valuable comments on 
the manuscript. 



References Cited 

Belton, P. 1967. Trapping mosquitoes with 
sound. Proc. Calif. Mosq. Contr. Assoc. 
35:98. 

Belton, P. and R. A. Costello. 1979. Flight 
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Belton, P. and R. H, Kempster. 1962. A field 
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Broch, J, T. 1969. Acoustic noise mea- 
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Costello, R. A. and R. A. Brust. 1976. Elec- 
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